Threatening letters calling for a “Punish a Muslim Day” on 3 April are being investigated by counter-terror police.
Muslims in London, Yorkshire and the Midlands have reported receiving the letter that calls for attacks in a points-scoring system.
The note incites verbal abuse and assaults on Muslims, as well as attacks on mosques.
West Yorkshire Police confirmed it had received six reports related to the letters.
The force said it had a couple of letters that will undergo analysis in an attempt to determine their origin.
Tell MAMA, a project that monitors Islamaphobic hate crimes, said a photo of an envelope suggests at least one of the notes was dealt with at a Sheffield sorting office.
UPDATE: We have received new reports of ‘Punish a Muslim Day’ letters in London and the Midlands.
We urge anyone who has received this letter to report to us in confidence or to ring the non-emergency 101 number.
We will continue to liaise with police on this matter.
— TellMAMAUK (@TellMamaUK) March 9, 2018
Iman Atta, the project’s director, said: “This has caused quite a lot of fear within the community. They are asking if they are safe, if their children are safe to play outdoors.
“We have told them to keep calm.”
In a statement, the organisation said: “We urge anyone who has received this letter to report to us in confidence or to ring the non-emergency 101 number.
“It is essential that all letters and envelopes are kept and handled minimally to preserve evidence for the police to investigate.
“Incidents like this are rare but we urge vigilance and calm as we remind Muslim communities that our confidential support service is available to assist in this matter.”
A West Yorkshire Police spokesman said: “Counter Terrorism Policing North East are coordinating the investigation at this time and will consider any potential links to existing enquiries.
“Anyone with any concerns about a communication they may have received should contact their local police force.”
A Metropolitan Police spokesman said it was not yet clear whether any criminal allegations relating to the letters had been reported in London.